Date of Award

Summer 1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Program/Concentration

Oceanography

Committee Director

John C. Ludwick

Committee Member

Chester E. Grosch

Committee Member

George F. Oertel

Abstract

In nearshore areas waves are generally irregular, and the irregular wave-induced currents have different peak velocities (magnitude asymmetry) and durations (duration asymmetry) between forward and backward motions. These asymmetries may produce a net cross-shore sediment transport in one direction. The sediment transport mostly occurs as bedload where the waves are non-breaking.

Sediment transport on a sloping bed is also affected by gravity, and accordingly the Shields parameter should be re-evaluated for a grain on a sloping bed. It was also found that the affect of a steady current that interacts nonlinearly with the waves was important for the cross-shore sediment transport and for the nearshore bottom morphology.

Both the numerical calculation and the near-bottom current measurements at Willoughby Spit show stronger peak velocities of shorter duration in the offshore direction in shallow water. This trend reversed with greater depth. The peak velocities were found to be more important than the complete velocity distribution for the net cross-shore sediment transport rate under oscillatory flow. The rate of sediment transport decreased exponentially with increasing water depth.

The calculation of cross-shore sediment transport rate and the analysis of magnitude asymmetry showed the existence of the depth of a neutral line (convergence). At depths shallower than the neutral depth sediment transport occurs in the offshore direction; the direction is onshore where the water depth is deeper than the neutral depth. Beach can reach a state of dynamical equilibrium by adjusting the water depth to the neutral depth corresponding to the local wave conditions.

At Willoughby Spit, the landwards shoreline retreat and the change of bathymetry after the beach-fill were accounted for by actions associated with the neutral depth. The change of the neutral depth as a result of wave-current interactions may very well explain the development of a characteristic trough and bar system within the groin compartment.

The bedload model of Madsen and Grant (1976), when incorporated with the irregular waves which were solved for by Biesel (1952), could reasonably well predict the overall bathymetric change after the fill at the study area.

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DOI

10.25777/c8w2-6h05

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